It has previously been proposed to determine the presence of gases by providing gas sensing units which contain gas sensing elements which change their electrical characteristics when the gas to be determined is present in the ambient atmosphere. Such gas sensing elements can provide an indication of the presence of carbon monoxide, methane, or specific gases such as butane or propane, as well as indicate the presence of gas mixtures, such as natural gas, coal gas, distillation gases, and the like. Other gas sensing elements are known which respond to toxic gases, vapors of organic solvents, combustion gases and the like. The change of the electrical characteristic of the sensing element occurs, e.g. due to interaction of the gas to be sensed with the element itself or for example based on catalytic oxidation of the gases which generate heat. Elements operating in this latter manner are e.g. the so-called pellistors. Various gas sensing elements are known which operate according to various physical and chemical principles, which may differ from each other; some gas sensing elements use metal oxide semiconductors. Gas sensing elements which can be used in the system of the present invention are described for example, in U.S. Pat. Nos. 3,695,848; 3,609,732; 3,676,820 and 3,644,795. The sensors there described change their electrical resistance when exposed to gases which can be oxidized, that is, to combustible gases.
Gas sensing units of whatever type are usually connected with connection lines to a central station. If the gas concentration exceeds a predetermined value, alarm devices are enabled by the central station to provide an alarm to personnel and, if desired, corrective counteraction may also be taken, for example by starting ventilators, exhaust apparatus, opening emergency exits, ventilation flaps, explosion suppression systems, water flooding systems, and the like.
Gas sensing systems as heretofore customarily used have the disadvantage that they may respond to "false alarms". The response thresholds of the gas sensing units must be set to be quite low, particularly when toxic gases are involved, so that corrective or emergency action can be taken even if the concentration of such gases is low or may occur only for a short period of time. Unnecessary, and hence undesired alarms may be given, however, due to spurious response of an element, random localized concentration of the gas to be sensed close to the sensing element, and the like; such false alarms interfere with effective use of the system and impair its reliability and its efficacy as an alarm unit.
It has previously been proposed--see German Patent No. 1,598,798--to use signaling systems having two different thresholds, in which, when the lower threshold is exceeded, a warning signal is provided and only when a second and higher threshold is reached, will an alarm signal be given. The circuit arrangement as there described is a combination of gas sensing element and evaluation circuit in form of a functionally single gas detection device. Such arrangements are not suitable for use as components in an overall gas sensing signaling system which is a composite of a plurality of gas sensing units and an evaluation or central station. If a plurality of such single devices are to be supervised, the respective devices have to be individually looked for the checked, and optical indication of which device has responded is not provided for. Further, if a device responds, and the response characteristics then disappear, it is impossible to locate which device has responded to correct any possible incipient malfunction in the elements supplying the gas which was sensed. By use of two thresholds, it is possible to set the alarm threshold higher than the warning threshold and thus to partially overcome the defect of prior systems of generation of unnecessary false alarms, at least in part. Yet, since the warning threshold then must be set lower than the previous single threshold, the generation of false warning signals becomes a greater factor.